Unlike animals such as a fox or a bear that are born with built-in furs, hu-man beings come into this world with little protection against the harsh en-vironmental conditions (Fig. 1–49). Therefore, we can claim that the search for thermal comfort dates back to the beginning of human history. It is be-lieved that early human beings lived in caves that provided shelter as well as protection from extreme thermal conditions. Probably the first form of heating system used was open fire, followed by fire in dwellings through the use of a chimney to vent out the combustion gases. The concept of cen-tral heating dates back to the times of the Romans, who heated homes by utilizing double-floor construction techniques and passing the fire’s fumes through the opening between the two floor layers. The Romans were also the first to use transparent windows made of mica or glass to keep the wind and rain out while letting the light in. Wood and coal were the primary en-ergy sources for heating, and oil and candles were used for lighting. The ru-ins of south-facing houses indicate that the value of solar heating was recognized early in the history.
The term air-conditioning is usually used in a restricted sense to imply cooling, but in its broad sense it means to condition the air to the desired level by heating, cooling, humidifying, dehumidifying, cleaning, and de-odorizing. The purpose of the air-conditioning system of a building is to provide complete thermal comfort for its occupants. Therefore, we need to understand the thermal aspects of the human body in order to design an ef-fective air-conditioning system.
The building blocks of living organisms are cells, which resemble minia-ture factories performing various functions necessary for the survival of organisms. The human body contains about 100 trillion cells with an aver-age diameter of 0.01 mm. In a typical cell, thousands of chemical reactions
*This section can be skipped without a loss in continuity.
Baby Bird
Fox
FIGURE 1–49
Most animals come into this world with built-in insulation, but human beings come with a delicate skin.
occur every second during which some molecules are broken down and en-ergy is released and some new molecules are formed. The high level of chemical activity in the cells that maintain the human body temperature at a temperature of 37.0°C (98.6°F) while performing the necessary bodily functions is called the metabolism. In simple terms, metabolism refers to the burning of foods such as carbohydrates, fat, and protein. The metabo-lizable energy content of foods is usually expressed by nutritionists in terms of the capitalized Calorie. One Calorie is equivalent to 1 Cal 1 kcal 4.1868 kJ.
The rate of metabolism at the resting state is called the basal metabolic rate, which is the rate of metabolism required to keep a body performing the necessary bodily functions such as breathing and blood circulation at zero external activity level. The metabolic rate can also be interpreted as the energy consumption rate for a body. For an average man (30 years old, 70 kg, 1.73 m high, 1.8 m2surface area), the basal metabolic rate is 84 W.
That is, the body is converting chemical energy of the food (or of the body fat if the person had not eaten) into heat at a rate of 84 J/s, which is then dissipated to the surroundings. The metabolic rate increases with the level of activity, and it may exceed 10 times the basal metabolic rate when some-one is doing strenuous exercise. That is, two people doing heavy exercising in a room may be supplying more energy to the room than a 1-kW resis-tance heater (Fig. 1–50). An average man generates heat at a rate of 108 W while reading, writing, typing, or listening to a lecture in a classroom in a seated position. The maximum metabolic rate of an average man is 1250 W at age 20 and 730 at age 70. The corresponding rates for women are about 30 percent lower. Maximum metabolic rates of trained athletes can exceed 2000 W.
Metabolic rates during various activities are given in Table 1–7 per unit body surface area. The surface area of a nude body was given by D.
DuBois in 1916 as
As 0.202m0.425h0.725 (m2) (1-30)
where m is the mass of the body in kg and h is the height in m. Clothing in-creases the exposed surface area of a person by up to about 50 percent. The metabolic rates given in the table are sufficiently accurate for most pur-poses, but there is considerable uncertainty at high activity levels. More ac-curate values can be determined by measuring the rate of respiratory oxygen consumption, which ranges from about 0.25 L/min for an average resting man to more than 2 L/min during extremely heavy work. The entire energy released during metabolism can be assumed to be released as heat (in sensible or latent forms) since the external mechanical work done by the muscles is very small. Besides, the work done during most activities such as walking or riding an exercise bicycle is eventually converted to heat through friction.
The comfort of the human body depends primarily on three environmen-tal factors: the temperature, relative humidity, and air motion. The temper-ature of the environment is the single most important index of comfort.
Extensive research is done on human subjects to determine the “thermal comfort zone” and to identify the conditions under which the body feels
1.2 kJ/s
1 kJ/s
FIGURE1–50 Two fast-dancing people supply more heat to a room than a 1-kW resistance heater.
comfortable in an environment. It has been observed that most normally clothed people resting or doing light work feel comfortable in the operative temperature (roughly, the average temperature of air and surrounding sur-faces) range of 23°C to 27°C or 73°C to 80°F (Fig. 1–51). For unclothed people, this range is 29°C to 31°C. Relative humidity also has a con-siderable effect on comfort since it is a measure of air’s ability to absorb moisture and thus it affects the amount of heat a body can dissipate by evaporation. High relative humidity slows down heat rejection by evapora-tion, especially at high temperatures, and low relative humidity speeds it up. The desirable level of relative humidity is the broad range of 30 to 70 percent, with 50 percent being the most desirable level. Most people at these conditions feel neither hot nor cold, and the body does not need to activate any of the defense mechanisms to maintain the normal body tem-perature (Fig. 1–52).
Another factor that has a major effect on thermal comfort is excessive air motion or draft, which causes undesired local cooling of the human body.
Draft is identified by many as a most annoying factor in work places, auto-mobiles, and airplanes. Experiencing discomfort by draft is most common among people wearing indoor clothing and doing light sedentary work, and least common among people with high activity levels. The air velocity should be kept below 9 m/min (30 ft/min) in winter and 15 m/min (50 ft/min) in summer to minimize discomfort by draft, especially when the air is cool. A low level of air motion is desirable as it removes the warm, moist air that builds around the body and replaces it with fresh air. There-fore, air motion should be strong enough to remove heat and moisture from the vicinity of the body, but gentle enough to be unnoticed. High speed air motion causes discomfort outdoors as well. For example, an environment at 10°C (50°F) with 48 km/h winds feels as cold as an environment at
7°C (20°F) with 3 km/h winds because of the chilling effect of the air motion (the wind-chill factor).
A comfort system should provide uniform conditions throughout the living space to avoid discomfort caused by nonuniformities such as drafts, asymmetric thermal radiation, hot or cold floors, and vertical temperature stratification. Asymmetric thermal radiation is caused by the cold sur-faces of large windows, uninsulated walls, or cold products and the warm surfaces of gas or electric radiant heating panels on the walls or ceiling, solar-heated masonry walls or ceilings, and warm machinery. Asymmetric radiation causes discomfort by exposing different sides of the body to sur-faces at different temperatures and thus to different heat loss or gain by radiation. A person whose left side is exposed to a cold window, for exam-ple, will feel like heat is being drained from that side of his or her body (Fig. 1–53). For thermal comfort, the radiant temperature asymmetry should not exceed 5°C in the vertical direction and 10°C in the horizontal direction. The unpleasant effect of radiation asymmetry can be minimized by properly sizing and installing heating panels, using double-pane win-dows, and providing generous insulation at the walls and the roof.
Direct contact with cold or hot floor surfaces also causes localized dis-comfort in the feet. The temperature of the floor depends on the way it is constructed (being directly on the ground or on top of a heated room, being made of wood or concrete, the use of insulation, etc.) as well as the floor TA B L E 1 – 7
Metabolic rates during various activities (from ASHRAE
Walking (on the level):
2 mph (0.89 m/s) 115
Handling 50-kg bags 235 Pick and shovel work 235–280
Miscellaneous Leisure Activities:
*Multiply by 1.8 m2to obtain metabolic rates for an average man. Multiply by 0.3171 to convert to Btu/h · ft2.
covering used such as pads, carpets, rugs, and linoleum. A floor tempera-ture of 23 to 25°C is found to be comfortable to most people. The floor asymmetry loses its significance for people with footwear. An effective and economical way of raising the floor temperature is to use radiant heating panels instead of turning the thermostat up. Another nonuniform condition that causes discomfort is temperature stratification in a room that ex-poses the head and the feet to different temperatures. For thermal comfort, the temperature difference between the head and foot levels should not ex-ceed 3°C. This effect can be minimized by using destratification fans.
It should be noted that no thermal environment will please everyone. No matter what we do, some people will express some discomfort. The thermal comfort zone is based on a 90 percent acceptance rate. That is, an environ-ment is deemed comfortable if only 10 percent of the people are dissatis-fied with it. Metabolism decreases somewhat with age, but it has no effect on the comfort zone. Research indicates that there is no appreciable differ-ence between the environments preferred by old and young people. Exper-iments also show that men and women prefer almost the same environment.
The metabolism rate of women is somewhat lower, but this is compensated by their slightly lower skin temperature and evaporative loss. Also, there is no significant variation in the comfort zone from one part of the world to another and from winter to summer. Therefore, the same thermal comfort conditions can be used throughout the world in any season. Also, people cannot acclimatize themselves to prefer different comfort conditions.
In a cold environment, the rate of heat loss from the body may exceed the rate of metabolic heat generation. Average specific heat of the human body is 3.49 kJ/kg · °C, and thus each 1°C drop in body temperature corre-sponds to a deficit of 244 kJ in body heat content for an average 70-kg man. A drop of 0.5°C in mean body temperature causes noticeable but ac-ceptable discomfort. A drop of 2.6°C causes extreme discomfort. A sleep-ing person will wake up when his or her mean body temperature drops by 1.3°C (which normally shows up as a 0.5°C drop in the deep body and 3°C in the skin area). The drop of deep body temperature below 35°C may dam-age the body temperature regulation mechanism, while a drop below 28°C may be fatal. Sedentary people reported to feel comfortable at a mean skin temperature of 33.3°C, uncomfortably cold at 31°C, shivering cold at 30°C, and extremely cold at 29°C. People doing heavy work reported to feel comfortable at much lower temperatures, which shows that the activity level affects human performance and comfort. The extremities of the body such as hands and feet are most easily affected by cold weather, and their temperature is a better indication of comfort and performance. A hand-skin temperature of 20°C is perceived to be uncomfortably cold, 15°C to be extremely cold, and 5°C to be painfully cold. Useful work can be per-formed by hands without difficulty as long as the skin temperature of fin-gers remains above 16°C (ASHRAE Handbook of Fundamentals, Ref. 1, Chapter 8).
The first line of defense of the body against excessive heat loss in a cold environment is to reduce the skin temperature and thus the rate of heat loss from the skin by constricting the veins and decreasing the blood flow to the skin. This measure decreases the temperature of the tissues subjacent to the skin, but maintains the inner body temperature. The next preventive
Clothing insulation (clo) The effect of clothing on the environment temperature that feels comfortable (1 clo 0.155 m2· °C/W 0.880 ft2· °F · h/Btu) (from ASHRAE Standard 55-1981).
23°C RH = 50%
Air motion 5 m/min
FIGURE 1–52 A thermally comfortable environment.
measure is increasing the rate of metabolic heat generation in the body by shivering, unless the person does it voluntarily by increasing his or her level of activity or puts on additional clothing. Shivering begins slowly in small muscle groups and may double the rate of metabolic heat production of the body at its initial stages. In the extreme case of total body shivering, the rate of heat production may reach six times the resting levels (Fig.
1–54). If this measure also proves inadequate, the deep body temperature starts falling. Body parts furthest away from the core such as the hands and feet are at greatest danger for tissue damage.
In hot environments, the rate of heat loss from the body may drop be-low the metabolic heat generation rate. This time the body activates the op-posite mechanisms. First the body increases the blood flow and thus heat transport to the skin, causing the temperature of the skin and the subjacent tissues to rise and approach the deep body temperature. Under extreme heat conditions, the heart rate may reach 180 beats per minute in order to main-tain adequate blood supply to the brain and the skin. At higher heart rates, the volumetric efficiency of the heart drops because of the very short time between the beats to fill the heart with blood, and the blood supply to the skin and more importantly to the brain drops. This causes the person to faint as a result of heat exhaustion. Dehydration makes the problem worse.
A similar thing happens when a person working very hard for a long time stops suddenly. The blood that has flooded the skin has difficulty returning to the heart in this case since the relaxed muscles no longer force the blood back to the heart, and thus there is less blood available for pumping to the brain.
The next line of defense is releasing water from sweat glands and resort-ing to evaporative coolresort-ing, unless the person removes some clothresort-ing and reduces the activity level (Fig. 1–55). The body can maintain its core tem-perature at 37°C in this evaporative cooling mode indefinitely, even in en-vironments at higher temperatures (as high as 200°C during military endurance tests), if the person drinks plenty of liquids to replenish his or her water reserves and the ambient air is sufficiently dry to allow the sweat to evaporate instead of rolling down the skin. If this measure proves inad-equate, the body will have to start absorbing the metabolic heat and the deep body temperature will rise. A person can tolerate a temperature rise of 1.4°C without major discomfort but may collapse when the temperature rise reaches 2.8°C. People feel sluggish and their efficiency drops consid-erably when the core body temperature rises above 39°C. A core tempera-ture above 41°C may damage hypothalamic proteins, resulting in cessation
Warm wall Radiation
Radiation Cold
window
FIGURE 1–53
Cold surfaces cause excessive heat loss from the body by radiation, and thus discomfort on that side of the body.
B r r r !
Shivering
FIGURE 1–54
The rate of metabolic heat generation may go up by six times the resting level during total body shivering in cold weather.
of sweating, increased heat production by shivering, and a heat stroke with irreversible and life-threatening damage. Death can occur above 43°C.
A surface temperature of 46°C causes pain on the skin. Therefore, direct contact with a metal block at this temperature or above is painful. How-ever, a person can stay in a room at 100°C for up to 30 min without any damage or pain on the skin because of the convective resistance at the skin surface and evaporative cooling. We can even put our hands into an oven at 200°C for a short time without getting burned.
Another factor that affects thermal comfort, health, and productivity is ventilation. Fresh outdoor air can be provided to a building naturally by doing nothing, or forcefully by a mechanical ventilation system. In the first case, which is the norm in residential buildings, the necessary ventilation is provided by infiltration through cracks and leaks in the living space and by the opening of the windows and doors. The additional ventilation needed in the bathrooms and kitchens is provided by air vents with dampers or ex-haust fans. With this kind of uncontrolled ventilation, however, the fresh air supply will be either too high, wasting energy, or too low, causing poor indoor air quality. But the current practice is not likely to change for resi-dential buildings since there is not a public outcry for energy waste or air quality, and thus it is difficult to justify the cost and complexity of me-chanical ventilation systems.
Mechanical ventilation systems are part of any heating and air condi-tioning system in commercial buildings, providing the necessary amount of fresh outdoor air and distributing it uniformly throughout the building. This is not surprising since many rooms in large commercial buildings have no windows and thus rely on mechanical ventilation. Even the rooms with windows are in the same situation since the windows are tightly sealed and cannot be opened in most buildings. It is not a good idea to oversize the ventilation system just to be on the “safe side” since exhausting the heated or cooled indoor air wastes energy. On the other hand, reducing the venti-lation rates below the required minimum to conserve energy should also be avoided so that the indoor air quality can be maintained at the required lev-els. The minimum fresh air ventilation requirements are listed in Table 1–8.
The values are based on controlling the CO2and other contaminants with an adequate margin of safety, which requires each person be supplied with at least 7.5 L/s (15 ft3/min) of fresh air.
Another function of the mechanical ventilation system is to clean the air by filtering it as it enters the building. Various types of filters are available
Another function of the mechanical ventilation system is to clean the air by filtering it as it enters the building. Various types of filters are available